Recently, there has been remarkable technological innovation in food packaging industry. For sanitary, beautiful appearance, and decreasing of weight to save carrying cost, for example it has been well known that thermoplastic resins such as polystyrene, polyvinyl chloride, polycarbonate and the like have been used in the applications of food packaging. Such hydrophobic thermoplastic resins, however, are not suitable for storage of foods, carbonated beverages, etc. for long period of time because of their less gas barrier properties against oxygen, carbon dioxide gas or the like. Especially, although impact-resistant polystyrene resin has good properties such as an excellent moldability, good balance of impact resistance and rigidity, good sanitation and the like, such polystyrene resin has insufficient gas barrier properties. Then, such polystyrene resin is not suitable for storage of foods for long period of time, either, and is limited within the applications of container for foods and packaging material. Accordingly, there have been still used many glass bottles and metal cans, and there have been many problems including nuisance of throwing away empty cans onto a road and much cost for recovery of bottles. Thus, there has been desired the development of an improved packaging and container material instead of conventional materials.
Since saponified ethylene-vinyl acetate copolymer having an ethylene content of 20-55 % by mole shows good moldability through melting, less gas permeability, high clarity, excellent gas barrier properties, such resin is superior in these characteristics for foods container and packaging material. However, it is not sufficient in other various characteristics, for example, less rigidity, less moisture resistance, and the like.
Then, it has been proposed that in order to obtain foods container or packaging material, which has an excellent gas barrier properties of saponified ethylene-vinyl acetate copolymer together with an excellent mechanical properties of polystyrene, these two kinds of resin layer are laminated. However, these two kinds of resin layer have hardly any affinity to each other, and hence, it is impossible to obtain a laminated article by a conventional simple thermal adhesion.
It has been known to produce a laminated resin article which is characterized in that polystyrene resin and resin having gas barrier properties, which have no adhesion with each other, are subjected to melt co-extrusion through the medium of an adhesive resin selected form the group of modified ethylene-vinyl acetate copolymer having unsaturated carboxylic acid or anhydride thereof in an amount of 0.001 to 10% by weight, or ionomer (cf. Japanese Laid Open Publication No. 46281/1979). The said reference discloses that as a gas barrier resin numerous resins such as saponified ethylene-vinyl acetate copolymer, polyamide resin, polyvinylidene chloride resin, thermoplastic polyester resins and the like can be laminated with polystyrene resin through the medium of an adhesive resin. The reference also discloses that such lamination can be carried out by melt co-extrusion process employing as an adhesive modified ethylene-vinyl acetate copolymer having unsaturated carboxylic acid or anhydride thereof in an amount of 0.001-10 weight % (modified EVA) or ionomer, and shows superiority thereof.
Further, Japanese Laid Open Publication No. 76366/1976 discloses that polyolefin can be laminated with saponified ethylene-vinyl acetate copolymer by using an ethylene-vinyl acetate copolymer modified by ethylenically unsaturated carboxylic acid or anhydride thereof under pressure in a melting condition. The said reference also discloses that there can be used by dilution of a modified ethylene-vinyl acetate copolymer with an ethylene-vinyl acetate copolymer, and shows superiority thereof.
However, when a polymer consisting of ethylene component-vinyl acetate component-ethylenically unsaturated carboxylic acid component or anhydride thereof are used as an adhesive resin layer for numerous laminated articles including various combinations of resins, it is usually observed that among the combination of layer (A) and layer (B) some adhesive resin can adhere to layer (A) at high strength and can not adhere to layer (B) at no strength, and vice versa. Accordingly, the property of adhesive resin layer is prepared due to a particular combination of layers to be subjected to lamination.
In the combination of saponified ethylene-vinyl acetate copolymer and polystyrene resin, some laminating process can be carried out according to the process in the aforementioned references, when lamination is carried out through compressing and heating by a hot press for long period of time, or through lamination by extruding for long period of time with extremely low rate of not more than 0.5 m/min. However, for co-extrusion lamination at the commercial operation rate, such conventional process can not produce a laminated article having a sufficient adhesion and value as a commercial product and induces troubles during molding operation.